The present invention relates generally to peristaltic pumps, and more particularly to a novel peristaltic pump having means for varying the occlusion rates of a compressible flow tube so as to optimize tube life.
Fluid flow pumps of the peristaltic type wherein moving regions of occlusion are effected on a compressible flow tube by rollers or thrust elements carried by a rotor are generally known. See, for example, U.S. Pat. No. 3,358,609 to Worth et al. Such pumps generally have reaction surfaces formed either intergral with the housing or on a separate housing or reaction member mounted such that the reaction surface lies in the plane of the circular path traversed by the compression rollers. In the operating mode, the reaction surface is substantially concentric with the circular path traversed by the compression rollers, and has an arcuate extent of substantially less than 360 degrees. The compressible flow tube is disposed between the reaction surface and the rotor so that the compression rollers compress or occlude the tube as they traverse their circular paths so as to create a peristaltic pumping action.
The flow rate of a peristaltic pump is a function of the rotor speed and the total volume of the occluded portion of the compressible flow tube. Thus, for a given pump having a predetermined compressible flow tube size and rotor speed, it is desirable that the compressible tube encompass or encircle as much of the 360 degree path traversed by the compression rollers as possible to achieve maximum flow capacity. While maximum flow is highly desirable, the overall efficiency of the pump, in terms of service life and operating costs in comparison to other types of pumps, is, in major part, a function of the life of the compressible tube. The service life of compressible tubes employed in peristaltic pumps of known prior design has been relatively short, requiring frequent replacement or maintenance as compared to maintenance intervals required for the operation of other types of pumps. Further, failure to adhere to a relatively stringent maintenance schedule frequently results in failure of the compressible tube, thus resulting in product loss. The maintenance requirements and risk of product loss should the operator fail to maintain a rigid maintenance schedule have been a deterrent to the wider adoption and use of peristaltic type pumps.
Another deterrent to the wider adoption of peristaltic type pumps is the relatively high cost of compressible tubes which are compatible with peristaltic pumps. The service life of a compressible tube employed in a peristaltic pump has been found to be inversely proportional to the amount of occlusion during a pumping mode, while pump performance in terms of the ability to develop pressure and vacuum, but not flow, has been found to be directly proportional to the amount of tube occlusion. It has thus been necessary in the design of peristaltic pumps to compromise tube life for the sake of pump performance.
The fixed occlusion rate of known peristaltic pumps also requires that the wall thickness of the compressible tubes be precise and consistent since the occlusion rate is dependent upon the combined wall thickness of the tube. For a given peristaltic pump, pump performance decreases when the wall thickness of the compressible tube is thinner than that which results in optimum performance, while tube life decreases with compressible tubes having greater wall thicknesses. Although compressible tubes having precise and consistent wall thickness result in more efficient pump operation, such tubes are more expensive than compressible tubes having variable wall thickness.
It has also been found that the tube life of a compressible tube employed in a peristaltic type pump is effected by a phenomenon wherein heat generated by compression or occlusion of the tube material tends to migrate in the direction of rotation of the thrust or compression rollers carried by the rotor. It has been found that a "hot spot" develops at the point on the tube where the compression rollers begin to release occlusion resulting in thermal degradation of a small area of the occluded section of the tube. If the hot spot is induced to reside or build up in a longer portion of the compressible tube, a corresponding decrease in maximum temperature results and thermal degradation is substantially decreased resulting in an increase in tube service life.